Orthogonal Frequency Division Multiplexing (OFDM) is a modulation technique which has found much favour in communication systems, such as for example those designed to operate in accordance with the first and second generation Digital Video Broadcasting terrestrial standards (DVB-T/T2) and is also being proposed for fourth generation mobile communication systems which are also known as Long Term Evolution (LTE). OFDM can be generally described as providing K narrow band sub-carriers (where K is an integer) which are modulated in parallel, each sub-carrier communicating a modulated data symbol such as Quadrature Amplitude Modulated (QAM) modulation symbol or Quaternary Phase-shift Keying (QPSK) modulation symbol. The modulation of the sub-carriers is formed in the frequency domain and transformed into the time domain for transmission. Since the data symbols are communicated in parallel on the sub-carriers, the same modulated symbols may be communicated on each sub-carrier for an extended period, which can be longer than the coherence time of the radio channel. The sub-carriers are modulated in parallel contemporaneously, so that in combination the modulated carriers form an OFDM symbol. The OFDM symbol therefore comprises a plurality of sub-carriers each of which has been modulated contemporaneously with a different modulation symbol.
In the Next Generation for Hand held (NGH) television system it has been proposed to use OFDM to transmit television signals from base stations disposed throughout a geographical area. In some examples the NGH system will form a network in which a plurality of base stations communicate OFDM symbols contemporaneously on the same carrier frequency thereby forming a so-called single frequency network. As a result of some of the properties of OFDM, a receiver may receive the OFDM signals from two or more different base stations which can then be combined in the receiver to improve the integrity of the communicated data.
Whilst a single frequency network has advantages in terms of operation and improved integrity of the communicated data, it also suffers a disadvantage if data local to a part of the geographical area is required to be communicated. For example, it is well known in the United Kingdom that the national carrier, the BBC, broadcasts television news throughout the entire national network but then switches, at certain times, to “local news” in which a local news programme is transmitted which is specifically related to a local area within the national network. However, the United Kingdom operates a multi-frequency DVB-T system so that the insertion of local news or local content of any sort is a trivial matter because the different regions transmit DVB-T television signals on different frequencies and so television receivers simply tune to an appropriate carrier frequency for the region without interference from other regions. However, providing an arrangement to insert data locally in a single frequency network presents a technical problem.
A known technique for providing a hierarchical or multi-layer modulation scheme in a single frequency OFDM network is disclosed in US 2008/0159186. The hierarchical modulation scheme provides a plurality of modulation layers which can be used to communicate data from different data sources or pipes contemporaneously.